Light emitting diodes (LEDs) are emerging as alternatives to classical lighting devices, such as fluorescent tubes and light bulbs, in many applications for lighting devices. The development of high-power LEDs with high light output has expanded the areas of use of LEDs for conventional lighting purposes.
Due to its construction and mechanism of function, an LED emits light within a rather narrow wavelength band, and conventional LEDs, for instance, emit light in the UV, blue, green, red or IR-band.
In conventional lighting applications, “white” light is often desired, and several approaches has been developed in order to convert the monochromic light of LEDs into white light. In one approach, light from red, green and blue emitting diodes is mixed into white light. In another approach, a light emitting diode, typically a blue emitting diode, is provided with a wavelength converting material that partially converts the emitted light into another color. For example, by providing a blue emitting LED with a yellow emitting wavelength converting material that converts a desired portion of the blue light into yellow, the mixture of unconverted, blue light and converted, yellow light results in a whitish light.
This latter approach of wavelength converted LEDs has proven as an attractive way of achieving the desired color of the output light.
There are several requirements on wavelength converting materials for use in these type of applications.
High power LEDs dissipate a lot of thermal energy while emitting high intensity light, and a wavelength converting material arranged on or near such an LED needs to be thermally stable and also light stable in order to ensure long life-time and consistent color throughout the life-time of the device.
Further, the degree of the wavelength conversion is dependent on the concentration of active substances in the wavelength converting material, and also on the thickness of the material. Hence, the thickness and the concentration should be able to be controlled accurately.
A ceramic wavelength converting element is described in US 2004/0145308 A1, the element being formed from a polycrystalline ceramic body of an yttrium aluminum garnet (YAG) which is doped with an activator, such as cerium.
However, the number of ceramic materials available for the manufacture of such wavelength converting elements are quite low, limiting the possibilities to fine-tune the wavelength conversion. Further, sintering of ceramic materials is performed at very high temperatures, which is detrimental to many luminescent materials, e.g. organic species.
In addition, accurate post production shaping of ceramic wavelength converting elements is difficult.
Thus there is a need for new materials for the manufacture of temperature and photo stable wavelength converting elements.